The present invention relates to a toroidal type continuously variable transmission to be used for, for example, automatic transmissions of automobiles.
Conventionally, there has been a toroidal type continuously variable transmission shown in FIG. 2. In this toroidal type continuously variable transmission, a first input disc 81, an output disc 82 and a second input disc 83 are coaxially arranged in this order. The first and second input discs 81, 83 and the output disc 82 rotate independently of each other. This transmission also has a first roller 85 and a second roller 86. The first roller 85 for transmitting power from the first input disc 81 to the output disc 82 is sandwiched between the first input disc 81 and the output disc 82. The second roller 86 for transmitting power from the second input disc 83 to the output disc 82 is sandwiched between the second input disc 83 and the output disc 82.
The first roller 85 rotates about a rotating shaft 84, an axis of which is crossing the rotating shaft of the two discs 81,82, while keeping in frictional contact with circumferential raceways 81A, 82A formed of concavely curved surfaces of the first input disc 81 and the output disc 82, respectively. Thereby, the first roller 85 transmits power from the first input disc 81 to the output disc 82. Also, the second roller 86 rotates about a rotating shaft 89, an axis of which is crossing the rotating shaft of the two discs 83, 82, while keeping in frictional contact with circumferential raceways 83A, 82A formed of concavely curved surfaces of the second input disc 83 and the output disc 82, respectively. Thereby, the second roller 86 transmits power from the second input disc 83 to the output disc 82.
As shown by solid line or one-dot chain line in FIG. 2, the larger an angle at which the rotating shafts 84, 89 of the first and second rollers 85, 86 are tilted from a posture axially perpendicular to the shafts of the discs 81, 82, 83, the larger a speed increasing ratio (speed reduction ratio) from the first, second input discs 81, 83 to the output disc 82.
In the above conventional toroidal type continuously variable transmission, it has been practiced to make the discs and the rollers from a high carbon bearing steel (for example, Japanese Industrial Standards (JIS) G4805 SUJ2).
However, in the toroidal type continuously variable transmissions, contact portions between discs and rollers undergo a strict lubricating condition of boundary lubrication, and temperatures of the discs and the rollers easily become high when the discs and the rollers spin and/or slide. As a result, the toroidal type continuously variable transmissions have had a drawback that lifetime of the discs and the rollers is short.
Also in the toroidal type continuously variable transmission, a pressure on contact surfaces between the discs and the rollers becomes high, for example, as much as about 4 GPa (gigapascal). Therefore, when lubricating oil is not sufficiently supplied to between the discs and the rollers because of high viscosity of the lubricating oil particularly at a low temperature (for example xe2x88x9240xc2x0 C.), an oil film is hardly formed between contact surfaces of the discs and the rollers. As a result, there has been a drawback that sticking between the discs and the rollers easily occur and/or damage of the disc raceway surfaces occur in the toroidal type continuously variable transmission.
An object of the present invention is therefore to provide a toroidal type continuously variable transmission which is superior in heat resistance and long in life, capable of preventing sticking between discs and rollers as well as damage of raceway surfaces even under low-temperature conditions or high-speed rotations, and thus achieving reliable power transmission at all times.
In order to achieve the above-mentioned object, the present invention provides a toroidal type continuously variable transmission including at least one input disc having a raceway of a concavely curved surface in a radial direction of one side of the input disc, an output disc having a raceway of a concavely curved surface in a radial direction of one side of the output disc placed in opposition to the raceway of the input disc, and a roller transmitting a power from the input disc to the output disc while keeping in frictional contact with both the raceway of the input disc and the raceway of the output disc, wherein at least one of the disc and the roller is made of a heat-resisting bearing steel which contains: C of both 0.8 wt % or more and 1.5 wt % or less; Si of both 0.5 wt % or more and 2.0 wt % or less; Mn of both 0.3 wt % or more and 2.0 wt % or less; Cr of both 1.3 wt % or more and 1.98 wt % or less; Mo of both 0.3 wt % or more and 1.0 wt % or less; a total of Si and Mo being 1.0 wt % or more; and the residual compositions being composed of iron and unavoidable impurities.
In an embodiment of the invention, the input disc, the output disk and the roller are made of the heat-resisting bearing steel.
According to the above embodiment, it has been found out that by making the discs and the roller of the toroidal type continuously variable transmission from the heat-resisting bearing steel having the above composition, high-temperature strength and hardness of the discs and the roller can be ensured, allowing longer life to be achieved as compared with the case in which the discs and the roller are made of high carbon bearing steel (JIS G4805 SUJ2).
For the heat-resisting bearing steel to be used in the present invention, the reasons for limiting composition ranges of its alloy components are given below:
C: At least 0.8 wt % C is necessary in order to reinforce martensite with C dissolved into a base metal and assure hardness of the steel after tempering as well as improve the rolling fatigue life characteristic. However, too large amounts of C cause enormous carbides to be generated, resulting in deterioration of the rolling fatigue life characteristic, hence an upper limit of C is 1.5 wt %;
Si: At least 0.5 wt % Si is necessary in order to particularly contribute to the suppression of hardness degradation after tempering, under the interaction of Si dissolved into a base metal with later-described Mo, as well as act as a deoxidizer in making a steel ingot. However, too large amounts of Si considerably deteriorate machinability and forgeability, hence an upper limit of Si is 2.0 wt %, but less than 1.0 wt % Si is more preferable;
Mn: At least 0.3 wt % Mn is necessary in order to enhance the toughness of martensite in base metal and improve the hardness and rolling fatigue life of steel material by improving the hardenability of steel. However, too large amounts of Mn considerably deteriorate the machinability, hence an upper limit of Mn is 2.0 wt %;
Cr: At least 1.3 wt % Cr is necessary in order to form a carbide and to improve high-temperature rolling fatigue life characteristic of steel. However, too large amounts of Cr cause the hardness after tempering to be deteriorated so that the high-temperature rolling fatigue life is conversely deteriorated, hence an upper limit of Cr is 1.98 wt %;
Mo: At least 0.3 wt % of Mo is necessary and more preferably 0.5 wt % of Mo is necessary in order to allow dissolution into a base metal and thereby improve the hardness after tempering and the rolling fatigue life characteristic. However, too large amounts of Mo block those effects from being exhibited while leading to an increased cost, hence an upper limit of Mo is 1.0 wt %;
Si+Mo: 1.0 wt % or more is necessary in order to obtain superior hardness and high-temperature rolling fatigue life characteristic after high-temperature tempering.
In an embodiment of the invention, the input disc and the output disc are made of the heat-resisting bearing steel, and the roller is made of ceramics.
According to the above embodiment, in the toroidal type continuously variable transmission of this invention, the roller made of ceramics and the discs made of the heat-resisting bearing steel rotate while keeping in frictional contact with each other. High heat is generated between the discs and the roller by this frictional contact. The rollers having smaller surface areas become higher in temperature than the input/output discs having larger surface areas which easily release heat. However, since ceramics superior in heat resistance are used for the rollers in this invention, even if the toroidal type continuously variable transmission is used under high load and/or high rotational speed, sticking between discs and roller or damage of raceway surfaces does not occur. Furthermore, even if enough oil is not supplied to between the disc and the roller due to an extremely low service temperature, the discs and the roller are unlikely to undergo seizure or damage, and thereby this transmission securely transmits a power at all times.